Organic light-emitting diodes ("OLEDs") have demonstrated potential for applications in flat-panel displays due to the OLEDs' high luminescent efficiency, low driving voltage, large viewing angle, light weight, simple device fabrication, and potential low cost. See generally, U.S. Pat. No. 5,707,745 to Forrest et al. A typical OLED structure comprises one or more layers of organic materials sandwiched between a transparent anode such as a thin film of indium tin oxide ("ITO") on a glass substrate and a metal cathode. When a direct current is applied between the anode and the cathode, holes and electrons are injected in the organic layers from the anode and cathode, respectively, and radiatively recombine, emitting light. Such an OLED structure and method of forming thereof are disclosed in PCT Publication No. WO 97/48139, directed to OLED structures and methods that allow for multiple colors to be integrated into a single substrate.
For color displays, red, green and blue emitters or "pixels" are typically required. Because of the size of the pixels in high resolution screens, it is impractical to deposit individual OLED pixels on a surface. However, the emission color of OLEDs can be changed through the incorporation of relatively small quantities of luminescent dyes into the host organic layers containing light-emitting materials. Therefore, closely positioned yet distinctly colored light emitters may be patterned by depositing a continuous layer of an OLED material on a surface followed by precise positioning and imparting small amounts of differently colored dyes to the layer. In other words, the color of an OLED display can be locally "tuned" by patterning the dye material without disturbing the OLED material. This concept has been demonstrated by introducing luminescent dyes locally using inkjet printing techniques. See, e.g., Hebner et al. (1998) Appl. Phys. Lett. 73: 1775-77. However, inkjet printing and other methods in which a separate dye droplet has to be applied for each individual pixel will limit the manufacturing rate of such displays. In addition, organic materials do not generally withstand conventional photolithographic processing in which solvents are used.
Thus, an improved method of introducing a dye pattern into an organic thin film over a large area, without resorting to sequential introduction of color into each individual pixel, is clearly desirable. One such method of patterning involves a hybrid stamp structure as described in U.S. Pat. No. 5,817,242 to Biebuyck et al. This patent describes a stamp is where a patterned layer is provided that can easily adhere or absorb a specific ink. If the ink contains an organic solvent, the stamp containing the solvent-based ink cannot be used to transfer the ink on to a receiving layer that may be adversely affected by the solvent.
Another method of introducing a dye pattern to an organic thin film over a large area is described in Pschenitzka et al. (1999), "Three-Color Organic Light-Emitting Diodes Patterned by Masked Dye Diffusion," Appl. Phys. Lett. 74: 1913-15. This reference describes diffusion by using a large area dye-doped polymer layer as the diffusion source. A patterned masking layer is sandwiched between the dye source plate and a substrate containing the receiving layer for OLED fabrication. By applying heat, the dye vaporizes or sublimes and thus deposits on and diffuses into the receiving layer in the desired pattern.
However, this thermal transfer method suffers from two main drawbacks. First, to effect dye transfer, the diffusion source must be heated to at least the vaporization or sublimation temperature of the dye. Because the dye source plate is in physical contact with a patterned mask which, in turn, is in contact with the film, thermal conduction would likely result in the receiving layer being heated to roughly the temperature of the diffusion source. Such excess heating will alter the microstructure or morphology of the film and may even damage the film. In particular, if the film already contains another dye pattern, that dye pattern will migrate due to elevated processing temperature and thus compromise the resolution of the final product. Second, because the mask necessarily has a thickness, the resulting resolution of the deposited dye pattern is further limited due to geometrical limitations on vapor transport from the dye source to the film.